Document the usage of each code block
Apply 'pre-wrap' only on code
Remove obsolete css styles
Follow css selector convention
Put related styles together
Tidy the css styles a bit
Move expander '<span>' to template
Reduce duplicate stage button code
Extract to 'exec_template' function
Extract to 'diff_hunk()' function
Move the wrapped html into template
Replace 'FILE_BODY' at the end

It rarely happens, if 'FILE_BODY' contains placeholders, it has to be
the last placeholder to be replaced.